Soil Vulnerability Assessment of Afikpo South Local Government 2 Area, Ebonyi State, Nigeria

1 SOIL VULNERABILITY ASSESSMENT OF AFIKPO SOUTH LOCAL GOVERNMENT 2 AREA, EBONYI STATE, NIGERIA 3 ABSTRACT 4 Remote Sensing and Geographic Information System (GIS) integrated with Revised Universal 5 Soil Loss Equation (RUSLE) was adopted to estimate the rate of annual soil loss in Afikpo South 6 Local Government. This is important due to the fact that agriculture is the main source of 7 livelihood in the area. The RUSLE factors were computed using data such as rainfall from 8 NIMET, Soil from FAO, elevation from SRTM and Landsat 8 OLI from USGS. The data were 9 used as input in a GIS environment and the annual soil loss was generated using the RUSLE 10 equation. The result shows that the average annual soil loss ranges from 0 to 155,858 ha/ton/yr. 11 It was also observed that soil erosion was predominant in the southern part of Afikpo South LGA 12 due to the presence of steep slopes in the area. The study serves as a preliminary documentation 13 for planning, conservation and management of soil resources in the Local Government. 14 Keywords: Soil Vulnerability, Estimation, RUSLE, Afikpo South, Nigeria 15 16 INTRODUCTION 17 Soil erosion is just the efficient expulsion of soil, including plant nutrients, from the land surface 18 by the different operators of denudation. It happens in several parts of Nigeria under various 19 land, climatic and soil conditions. Be that as it may, the level of event shifts extensively from one 20 part of the nation to the other (Ofomata, 2015). Similarly varied are the components in charge of 21 the commencement and advancement of erosion, just as the sorts that exist in several parts of the 22 country. 1 23 Soil erosion is a major ecological problem in southeast Nigeria (Abegunde et al., 2006). Soil 24 erosion leads to soil deterioration, and this coupled with improper landuse practices becomes 25 increasingly devastating (Feng et al., 2010). Massive and expanding gully erosion are prevalent 26 in five states which include Anambra, Imo, Abia, Ebonyi and Enugu. The root causes of the 27 gully erosion are complex with climate change amplifying the challenge (Worldbank, 2013). 28 Erosion menace has destroyed buildings, farmlands, landmarks, ancestral burial sites, economic 29 trees and Social infrastructures. Soil erosion pose a serious threat to food production. In Afikpo 30 South Local Government Area of Ebonyi state, there is decrease in agricultural productivity due 31 to soil erosion effect. The situation has become critical with increase in population, urbanization, 32 deforestation and intensified rainfall. There is a need for policies that encourages soil retention 33 strategies, land improving investments and better land management (Scherr and Yadav, 1997). 34 Causes of gully erosion has been traced to poor land management coupled with lack of 35 innovation and awareness measure (Obidimma and Olorunfemi, 2011). 36 MATERIALS AND METHODS 37 STUDY AREA 38 Afikpo South Local Government Area (LGA) is located between longitude 7°40’ E to 7° 53’ E 39 and latitude 5° 41’ N to 5° 57’ N. Afikpo South is historically known as Edda. It is the homeland 40 of Edda people an Igbo subgroup. It administrative headquarters is at Nguzu Edda. Edda is 41 bordered by Unwana to the east, Akaeze to the west, Ohafia to the south and Amasiri to the 42 north. The basic occupation in the LGA is farming and fishing. Afikpo South LGA covers a total 43 of 330km2 in area with a population of 157,072 according to the 2006 Nigeria population census. 44 The climate is characterised by two distinct seasons; the dry and rainy season. The rainy season 45 is oppressive and overcast. The dry season is muggy and mostly cloudy. According to Koppen 2 46 and Geiger, the climate of Afikpo is classified as Aw with average temperature of 27.2°C and 47 average annual rainfall of 2022mm. Afikpo South landscape features lush vegetation with 48 parklands and short tress added to a litter of woodlands and forests. 49 50 Figure 1: Map of Afikpo South LGA-Study Location 51 DATA 52 Rainfall data for Afikpo South LGA was acquired from the Nigerian Meteorological Agency 53 (NIMET) for the period of 1985 to 2015. The rainfall data was analysed to derive the average 54 annual rainfall for the LGA. 55 Landsat 8 imagery was acquired from the United States Geological Survey (USGS) imagery 56 distribution platform. The landsat imagery was acquired on 15th of January, 2015. The study area 3 57 was extracted from the large dataset. The imagery was used to generate the vegetation cover for 58 Afikpo South LGA. 59 Shuttle Radar Topography Mission (SRTM) imagery was acquired from the United States 60 Geological Survey (USGS) distribution platform. The SRTM image was used to delineate the 61 slope length and slope steepness of Afikpo South LGA. 62 Soil data was acquired from the Harmonised World Soil Database (HWSD) by Food Agriculture 63 Organisation (FAO). 64 METHODOLOGY 65 SOIL EROSION MODELLING BY UNIVERSAL SOIL LOSS EQUATION (USLE) 66 The Universal Soil Loss Equation (USLE) is an empirically based soil erosion model developed 67 by Wischmeier and Smith (1978). The soil loss (A) due to water erosion per unit area per year 68 (Mg ha-1 yr-1) was quantified using USLE following equation: 69 A = R x K x LS x C x P 70 Where A is the average soil loss due to water erosion, R the rainfall and runoff erosivity factor 71 (MJ mm ha-1 h-1 yr-1), K the soil erodibility factor (Mg h MJ-1 mm-1), L the slope length factor 72 (dimensionless), S the slope steepness factor (dimensionless), C the cover and management 73 practice factor (dimensionless), and P the support practice (dimensionless). 74 RAINFALL-RUNOFF EROSIVITY FACTOR (R) 75 The rainfall erosivity factor (R) is a representation of the potential of rainfall to erode soil 76 particles (Renard et al., 1997). The R factor relects the effect of long term average of rainfall 77 intensity on soil erosion [7]. 78 SOIL ERODIBILITY FACTOR (K) 79 Soil erodibility factor (K factor) is the soil loss rate per erosion index unit for a specified soil as 80 measured on a standard plot, which is defined as a 72.6ft (22.1m) length of uniform 9% slope in 81 continuous clean-tilled fallow (Ahmad, 2013). Soil erodibility factor measures the susceptibility 4 82 of soil particles or surface materials to transportation and detachment by the amount of rainfall 83 and runoff input [8]. K factor takes into account the integrated effect of rainfall, runoff and 84 infiltration on soil loss [9]. K factor quantifies the cohesive character of a soil type and its 85 resistance to dislodging and transport due to raindrop impact and overland flow shear forces 86 (Tirkey et al., 2013). 87 TOPOGRAPHY FACTOR (LS) 88 L factor is the function of slope length and S factor is the function for slope steepness. The both 89 which is written as LS factor is used to represent the topographical factor effect on soil erosion. 90 The effect of topography on soil loss is accounted for by the LS factor. Slope length is defined as 91 the distance from the point of origin of overland flow to either the point where the slope 92 decreases to the extent that deposition begins or the point where runoff enters well-defined 93 channels [7]. The slope length factor determines the concentration of water. The greater the slope 94 length of a field, the greater the concentration of water and run off (Nnabugwu and Ibeabuchi, 95 2015). 96 COVER MANAGEMENT FACTOR (C) 97 The cover and management factor indicates how vegetation cover, cropping and management 98 practices affect soil erodibility. Cover Management Factor (C) is the ratio of soil loss from a 99 particular site with a specified cover and management to soil loss from a standard unit plot [9]. 100 This simply means the ratio of soil loss of a specific crop to the soil loss under the condition of 101 continuous bare fallow [8]. This value includes the effects of cover, crop sequence, productivity 102 level, length of growing season, tillage practices, residue management, and the expected time 103 distribution of erosive rainstorms. 104 CONSERVATION PRACTICE FACTOR (P) 105 Support practice factor is the soil loss ratio with a specific support practice to the corresponding 106 soil loss with up and down slope tillage [8]. Farmers tend to plough their farmlands without 107 engaging in soil conservation practices such as contouring, stipping and terracing and this leads 108 to higher P value. Lower P values would be attained if conservation practices are been carried 109 out by farmers. These practices principally affect erosion by modifying the flow pattern, grade, 5 110 or direction of surface runoff and by reducing the amount and rate of runoff (Nwakor et al., 111 2015). 112 Rainfall Rainfall Raster R 113 114 115 116 117 Landsat 8 Vegetation Cover C Image 118 119 120 Soil Loss 121 SRTM Image Slope Contributing areas 122 and steepness LS 123 Stream 124 125 P 126 Soil Map Soil Erodibility raster 127 K 128 129 Figure 2: RUSLE Flow Chart 130 RESULTS AND DISCUSSION 131 RUSLE EROSION MODELLING 6 132 RAINFALL-RUNOFF EROSIVITY FACTOR (R) 133 NIMET precipitation data for 1985 – 2015 was acquired for Afikpo South LGA. The average 134 precipitation was calculated for the 30 years period. The average precipitation in Afikpo South 135 LGA for the 30 years period is 2385.39mm. The calculated average precipitation value was fitted 136 into the R factor formula: 137 R = 0.5 * P * 1.73 (in Metric unit) (Roose (1975) 138 The R factor was calculated as 2063.36 and this represents the rainfall regime within Ebonyi 139 state.

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